1. Field of the Invention
The present invention generally relates to methods for fabricating phosphor screens for cathode ray tubes (CRTs) and, more particularly, is directed to a method for drying phosphor slurries after having coated the phosphor slurries on a panel of the CRT.
2. Description of the Prior Art
An ordinary color CRT for a color television receiver is constituted by a glass bulb formed of a panel and a funnel, a phosphor screen, color selection electrodes and an electron gun or the like. In an ordinary color CRT for a color television receiver, for example, a CRT of a type shown in FIG. 1 employing an aperture grill as color selection electrodes, a glass panel 12 has a side wall (i.e., so-called shirt portion) 12a which extends from a panel portion opposing to an image being displayed toward the side wall of the glass panel. In this type of the CRT, a length L1 of the side wall 12a is about 20 mm. In contrast, a CRT of a so-called beam index type mounted in a vehicle or a cockpit of an airplane, which is high in luminance, not influenced by earth magnetism and has vibration proof characteristics, employs an index stripe or pattern instead of an aperture grille as color selection electrodes. A glass panel 2 of thebeam index type CRT has a side wall or skirt portion 2a whose length L2 is about 5 mm as shown in FIG. 2A, which is shorter than the skirt of the ordinary CRT shown in FIG. 1.
Accordingly, there arises a difference between the ordinary CRT and thebeam index type CRT as described bellow in a process for rotating the panel for drying phosphor slurry by using a far-infrared radiation heater after the coating of the phosphor slurry on the panel. That is, in a case of the ordinary CRT, since the length L1 of the skirt portion 12a is sufficiently long, the drying process of the phosphor slurry can be completed without causing an unnecessary or excessive phosphor slurry 14 to flow on a phosphor screen A as shown in FIG. 1 even when the panel is rotated at a speed between 20 and 40 r.p.m.
However, in a case of thebeam index type CRT, since the length L2 of the skirt portion 2a is short, the unnecessary phosphor slurry 14 is not completely disposed on the outer portions of the side wall but a part of the unnecessary phosphor slurry 14 flows on a phosphor screen A as shown in FIG. 2B when the panel is rotated at a speed between 20 and 40 r.p.m. like the ordinary CRT. If the rotational speed of the panel is increased to a range of 70 to 150 r.p.m. in order to prevent the unnecessary phosphor slurry from flowing on the phosphor screen, a phosphor slurry of a thick stripe configuration is formed at the center portion of the panel as shown in FIGS. 3 and 4, which results in a defect of vertical stripes 15 on the phosphor screen. In FIG. 4, reference numeral 17 denotes a light absorbing pattern of carbon with a stripe shape and 18 a phosphor pattern of stripe shape. This defect is inconspicuous when only one of the phosphor slurries of red, green and blue is coated on the panel. However, when phosphor slurries corresponding to red, green and blue are coated on the panel, the defect becomes conspicuous such that a vertical stripe line caused by unevenness of luminance due to the presence of the stripe phosphor slurry 15 appears on the screen when the resultant CRT is illuminated. Conventionally, this vertical stripe phosphor slurry has been eliminated by swinging or varying the rotational speed in a range of 20 to 40 r.p.m. This conventional method, however, is disadvantageous in that the mechanism for swinging the rotational speed is expensive.